On Sun, 31 Jan 2016 18:02:38 +0100 Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Harald Fuchs <hari.fuchs@gmail.com> writes: > > Ben Leslie <benno@benno.id.au> writes: > >> "Technically, PRIMARY KEY is merely a combination of UNIQUE and NOT NULL" > >> > >> I wanted to clarify if that was, technically, true. > > > Yes, but see below. > > >> "identifying a set of columns as primary key also provides metadata > >> about the design of the schema, as a primary key implies that other > >> tables can rely on this set of columns as a unique identifier for > >> rows." > > Yeah. The extra metadata has several other effects. Perhaps it would be > better to reword this sentence to make it clear that PRIMARY KEY is > equivalent to UNIQUE+NOTNULL in terms of the data constraint that it > enforces, without implying that there is no other difference. I'm not > sure about a short and clear expression of that though ...
How about:
"PRIMARY KEY is merly a combination of UNIQUE and NOT NULL with regard to data consistency behavior." "identifying a set of columns as primary key also provides metadata about the design of the schema, as a primary key implies that other tables can rely on this set of columns as a unique identifier for rows. This metadata may be used by external programs, but is also utilized interally by the server in some cases."
Do we have to be so vague...
A PRIMARY KEY enforces a UNIQUE, NOT NULL constraint and additionally allows the server to exhibit additional behaviors based upon the additional knowledge that the chosen constraint unique identifies a specific record. The behaviors are: <list them here>. External programs and extensions may also make use of the additional meta-data communicated through the use of PRIMARY KEY instead of a simple UNIQUE+NOTNULL constraint.